This invention relates to optical information media, typically read-only optical disks and optical recording disks, and a method for preparing the optical information media.
To record and store a vast quantity of information as typified by moving image information, advanced optical information media such as read-only optical disks and optical recording disks are required to increase their recording density for increasing the capacity. To meet such a demand, engineers have been engaged in the research and development works targeting a higher recording density.
One such approach relating to digital versatile disks (DVD) is to shorten the wavelength of a write/read laser beam and increase the numerical aperture (NA) of a write/read optical system objective lens, thereby reducing the spot diameter of the write/read laser beam. As compared with CD, DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the write/read wavelength from 780 nm to 650 nm and the NA from 0.45 to 0.6.
Increasing the NA, however, leads to a reduced tilt margin. The tilt margin is a permissible tilt of an optical information medium relative to an optical system, which depends on the NA. The tilt margin is in proportion to
xcex/(txc2x7NA3)
wherein xcex denotes the wavelength of write/read light and 35 xe2x80x9ctxe2x80x9d denotes the thickness of a transparent substrate the write/read light enters. If the optical information medium is inclined or tilted relative to the laser beam, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration is represented by
(xc2xd)xc2x7txc2x7{n2xc2x7sin xcex8xc2x7cos xcex8}xc2x7NA3/(n2-sin2xcex8)xe2x88x925/2
wherein n denotes the refractive index of the substrate and xcex8 is a tilt angle. It is appreciated from these formulae that the tilt margin may be increased and the occurrence of comatic aberration be suppressed by reducing the thickness xe2x80x9ctxe2x80x9d of the substrate. In fact, the DVD design is such that a tilt margin is secured by reducing the thickness of the substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD substrate. On the other hand, the thickness variation margin of the substrate is represented by
xcex/NA4.
If the substrate entails a thickness variation, a further wavefront aberration (or spherical aberration) occurs. The coefficient of spherical aberration is represented by
{(n2xe2x88x921)/8n3}xc2x7NA4xcex94t
wherein At denotes the thickness variation of the substrate. It is appreciated from these formulae that the thickness variation must be minimized before the spherical aberration can be suppressed when the NA is large.
Specifically the thickness variation xcex94t of DVD is limited to xc2x130 xcexcm whereas CD allows for a thickness variation xcex94t of xc2x1100 xcexcm.
To record moving images of better quality for a longer period of time, there has been proposed a structure allowing for use of a thinner substrate. In this structure, a substrate of an ordinary thickness is used as a support substrate for maintaining rigidity, pits or a recording layer is formed on the surface of the support substrate, and a light transmissive layer of about 0.1 mm thick is formed thereon as a thin substrate. Write/read light enters the pits or recording layer through the light transmissive layer. This structure can achieve a higher recording density due to a greater NA because the substrate can be made extremely thin as compared with the prior art.
However, the light transmissive layer used in the above structure was very difficult to form by injection molding of resin. It was then desired to have an effective method for forming such a light transmissive layer. For example, JP-A 9-161333 proposes to form a light transmissive layer by spin coating a UV-curable resin. In JP-A 10-269624, a light transmissive layer of uniform thickness is formed by dispersing spacer particles in a photo-curable resin, coating the resin onto a substrate, and pressing a plate against the coating.
The light transmissive layer formed by these methods has the problem that curing of the resin of which the light transmissive layer is formed necessarily entails contraction which causes the medium to bow or warp. If the photo-curable resin is applied as a coating of about 0.1 mm thick, it is difficult to accomplish uniform cure in a thickness direction. Then the light transmissive layer loses optical homogeneity, and the uncured monomer can reduce the reliability of the medium.
Besides, JP-A 10-283683 discloses the bonding of a light transmissive sheet with a UV-curable resin. The method of this patent involves first feeding a UV-curable resin to a substrate surface or a substrate surface and a surface of a light transmissive sheet, placing the light transmissive sheet on the substrate, and rotating the substrate and the light transmissive sheet together for causing the UV-curable resin to spread therebetween. Allegedly, this method precludes deformation of the sheet or spread-out of the adhesive to the readout area, and facilitates to form a light transmissive layer of uniform thickness and a very thin adhesive layer. The method is thus effective for precluding the deformation of the optical recording medium due to initial bowing of the substrate and aging.
JP-A 10-283683 using a UV-curable resin as the adhesive layer has the advantage of reduced bowing as compared with the above-described other methods, because the UV-curable resin layer is thin.
However, the method of JP-A 10-283683 involves several thousands of revolutions of the substrate with the light transmissive sheet rested thereon, during which the sheet can flutter. Then bubbles can be introduced into the adhesive layer to detract from its optical homogeneity. Additionally, the adhesive layer has a greater thickness distribution, especially in a circumferential direction of the disk. In the case of solvent diluted resin, evaporation of the solvent is rather discouraged.
JP-A 10-283683 lacks illustrative examples of the UV-curable resin. Although it is disclosed that the preferred viscosity of the UV-curable resin is in the range of 1 to 1,500 centipoise and the preferred thickness of the adhesive layer is in the range of 0.01 to 10 xcexcm, the publication lacks any illustrative example of forming an adhesive layer of a particular thickness using a UV-curable resin having a particular viscosity.
An object of the invention is, in conjunction with an optical information medium having an information recording means on one surface of a support substrate and a light transmissive layer of resin on the support substrate surface, to make the thickness of the light transmissive layer uniform. Another object is to preclude the bowing of such an optical information medium and to minimize the optical heterogeneity, especially birefringence increase of the light transmissive layer.
In a first aspect, the invention provides an optical information medium comprising a support substrate having an information recording means on one surface thereof, and a light transmissive layer on the support substrate surface, wherein writing and/or reading light enters the information recording means through the light transmissive layer. The light transmissive layer includes a light transmissive sheet composed of a resinous material and an adhesive layer for joining the light transmissive sheet to the support substrate, the adhesive layer comprising a cured product of a UV-curable resin and having an average thickness from 0.5 xcexcm to less than 5 xcexcm.
Preferably, the light transmissive sheet is composed of a polycarbonate, polyarylate or cyclo-olefin polymer. Also preferably, the light transmissive sheet has been formed by casting the resin.
In a second aspect, the invention provides a method for preparing an optical information medium comprising a support substrate having an information recording means on one surface thereof, and a light transmissive layer on the support substrate surface, wherein writing and/or reading light enters the information recording means through the light transmissive layer, the light transmissive layer comprising a light transmissive sheet composed of a resinous material and an adhesive layer for joining the light transmissive sheet to the support substrate, said adhesive layer comprising a cured product of a UV-curable resin. The method involves the steps of applying the UV-curable resin or a solution thereof onto the entire surface of said support substrate to form a coating; placing the light transmissive sheet on the coating, preferably in a reduced pressure atmosphere; and curing the UV-curable resin into the adhesive layer by irradiating ultraviolet radiation to the coating.
Preferably, the UV-curable resin or the solution thereof has a viscosity of less than 10 centipoise and a solid concentration of 10 to 50% by weight. Optionally interposed between the placing step and the curing step is the pressurizing step of applying a pressure to the surface of the light transmissive sheet, preferably by feeding a gas under pressure over the light transmissive sheet.